1. Field of the Invention
The present invention relates in general to electric motors and, in particular, to a diagnostic methodology for the recognition of the mutual positioning of a rotor and a stator in an electric motor, specifically an electric motor without brushes (brushless) with internal magnets, such as for example a drive motor for a vehicle.
2. Description of the Related Art
An electric motor, such as an electric starter motor or a drive motor for an electric or hybrid vehicle, is conventionally equipped with a device for sensing the angular position or resolver, adapted to determine the mutual angular position between a rotary shaft and a stationary portion of the motor, which corresponds to the mutual angular position between a rotor and a stator.
The information on angular position between rotor and stator is indispensable in the control of an electric motor, so that a unit for controlling the motor including an inverter circuit for generating the excitation currents of the motor is able to control the injection of current into the motor in the correct manner, controlling the rotation of the motor in the desired direction.
The resolver is an analogue transducer of angular position, which comprises a mobile part, associated with the rotor or with the rotary shaft of the electric motor, and a fixed part, associated with the stator or with another stationary portion of the electric motor. The resolver comprises an excitation winding through which a sinusoidal excitation current flows (with an angular frequency ω higher than the angular velocity), and two stationary windings (rigidly attached to the fixed part) in electrical phase quadrature. The excitation winding may be rigidly attached to the mobile part, or this may also be accommodated on the fixed part, if the mobile part has at least one pair of magnetic poles.
The principle of operation of the resolver is as follows: the mobile part set in rotation induces in the fixed windings an e.m.f. composed of two components: a first component of transformer-type origin, due to the variations in an excitation voltage Vr, and a second component due to the relative motion of the mobile part with respect to the fixed windings, which is proportional to the sine or to the cosine of the angle θ identified by the position of the mobile part with respect to a predetermined reference. Choosing θ=0 when the excitation winding or a particular magnetic pole of the mobile part is aligned with one of the two fixed windings, the expressions for the voltage signals Vs1 and Vs2 at the ends of the fixed windings are respectively:Vr(t)=VR sin(ωt)vs1=VR·sin(ω·t)·cos(θ)vs2=VR·sin(ω·t)·sin(θ)
The sine and the cosine of the angular position of the mobile part modulate in amplitude the carrier with an angular frequency ω present on the winding excitation. From the voltage signals Vs1 and Vs2 at ends of the fixed windings, by demodulation, it is possible to obtain an estimate of the angle θ.
In the production line of an electric motor, the mobile part and the fixed part of the resolver are stably attached to the motor in a random, undetermined position or—as an alternative, if a controlled procedure for assembly of the resolver with respect to the stator of the motor is provided—in a position close to a nominal value, taking into account the assembly tolerances, whereby a difference is established between the reference position of the resolver and a reference position of minimum reluctance of the electric motor, commonly known as offset of the resolver.
This position is measured at the end of the assembly line of an electric motor and is stored in memory within the control unit of the motor as the predetermined offset of the resolver. One representative example of a method for calibrating the offset of the resolver is disclosed in published US Application 2014/015457.
In normal operation of the electric motor, the control unit is capable of determining the correct angular position of the motor (the mutual angular position between stator and rotor) based on the output signal of the resolver knowing the offset of the resolver.
During the normal operation of an electric motor, it is possible for anomalies to occur, for example on the rotor, in such a manner as to modify the offset of the resolver with respect to the predetermined one imposed at the production site.
If the control unit of the motor does not recognize a modification of the offset of the resolver, the latter is no longer capable of determining the correct angular position of the motor or the position of minimum reluctance of the motor, which causes a lower torque to be provided or creates even serious drawbacks, such as unexpected behaviours in acceleration and deceleration when the vehicle is being driven, and—as a result of the erroneous control of the motor—the rupture of the rotor which determines the blocking of the motor drive shaft or the rupture of the pinion.
Equally disadvantageously, in the case of use of the electric motor in the start phase for the drive, the erroneous interpretation of the position of the rotor by the control unit may cause the inversion of the sign of the torque applied to the drive shaft with respect to that requested, causing the movement of the vehicle in the opposite direction to that desired, with consequent serious risks for the people on board the vehicle or around it.